The invention relates to ink compositions for ink jet printers which contain a magneto-rheological fluid.
The use of controllable fluids, electro-rheological (ER) and magneto-rheological (MR) fluids, in dampers, was first proposed in the early 1950's by Winslow in U.S. Pat. No. 2,661,596. The use of controllable fluids was generally restricted to the area of clutches, with a few exceptions, until the mid-1980's.
Magneto-rheological fluids (MR fluids) comprise a carrier medium, such as a dielectric medium (mineral oil or silicone oil) and solid particles that are magnetizable so as to respond to a magnetic field. These solid magnetizable particles comprise powders of iron, steel, nickel, cobalt, ferrites and garnets having a particles size large enough to incorporate a multiplicity of magnetic domains. As a result, the particles possess little or no permanent magnetic moment but are readily magnetized by an applied magnetic field. When an external magnetic field is applied to an initially random arrangement of magnetizable particles, a magnetic moment (roughly) parallel to the applied field is induced in each particle. The force between two particles whose moments are aligned head-to-tail is attractive, promoting the formation of chains of nearly contacting particles aligned along the direction of the field. The magneto-rheological fluid fibrillates and highly elongated structures of particles form parallel to the field, typically within milliseconds. The elongated structures of particles essentially form a solid in that the MR fluid does not flow at low shear stress. At high shear stress, the MR fluid flows like a viscous liquid. The stress at which the chains are disrupted and the particles flow is referred to as the yield stress. The yield stress is a function of the magnitude of the applied magnetic field.
The basis for the magneto-rheological effect can be explained by the interparticle force induced by the applied magnetic field.
Ink jet printing has experienced a significant increase in use in recent years due to reduced equipment cost, color availability and improvements in print speed and print resolution. Conventional ink jet printers operate by employing a plurality of actuator elements to expel droplets of ink through an associated plurality of nozzles. A print head actuator is described in U.S. Pat. No. 4,516,140. Each actuator element is typically located in a chamber filled with ink supplied from a reservoir and each actuator element is associated with a nozzle that defines part of the chamber. On energizing a particular actuator element, a droplet of ink is expelled through the nozzle toward a receiving substrate.
There are two major categories of ink jet printing--"Drop-on-Demand" and "continuous" ink jet printing. For continuous ink jet printing, a conducting ink is supplied under pressure to an ink nozzle and forced out through a small orifice. Prior to passing out of the nozzle, the pressurized ink stream proceeds through a ceramic crystal which is subjected to an electric current. This current causes a piezoelectric vibration equal to the frequency of the AC electric current. This vibration, in turn, generates the ink droplets from the unbroken ink stream. The ink stream breaks up into a continuous series of drops which are equally spaced and of equal size. Surrounding the jet, at a point where the drops separate from the liquid stream in a charge electrode, a voltage is applied between the charge electrode and the drop stream. When the drops break off from the stream, each drop carries a charge proportional to the applied voltage at the instant at which it breaks off. By varying the charge electrode voltages at the same rate as drops are produced it is possible to charge every drop to a predetermined level. The drop stream continues its flight and passes between two deflector plates which are maintained at a constant potential. In the presence of this field, a drop is deflected towards one of the plates by an amount proportional to the charge carried. Drops which are uncharged are undeflected and collected into a gutter to be recycled to the ink nozzle. Those drops which are charged, and hence deflected, impinge on a substrate traveling at a high speed at right angles to the direction of drop deflection. By varying the charge on individual drops, the desired pattern can be printed.
In a typical "Drop-on-Demand" ink jet printing process, a fluid ink is forced under pressure through a very small orifice of a diameter typically about 0.0024 inches in the form of minute droplets by rapid pressure impulses. The rapid pressure impulses are typically generated in the print head by either expansion of a piezoelectric crystal vibrating at a high frequency or volatilization of a propellant within the ink by rapid heating cycles. The piezoelectric crystal expansion causes the ink to pass through the orifice as minute droplets in proportion to the number of crystal vibrations. Thermal jet printers employ a heating element within the print head to volatilize a propellant and form droplets in proportion to the number of on-off cycles for the heating element. The ink is forced out of the nozzle when needed to print a spot on a substrate as part of a desired image. The minute droplets may be energized to achieve an electrical charge and deflected as in the continuous ink jet printing. Conventional ink jet printers are more particularly described in U.S. Pat. No. 3,465,350 and U.S. Pat. No. 3,465,351.
Less common Drop-on-Demand ink jet printing processes control the discharge of ink and the amount thereof from a static or variable pressure through the nozzle by varying the viscosity of the ink using an electro-rheological effect. Unlike the processes of U.S. Pat. No. 5,300,969 where inks for jet printing are heated to reduce their viscosity, these methods increase the viscosity of the ink. These less common drop-on-demand processes require a special configuration for the print head to apply an electric potential to the ink so as to vary its viscosity. Examples of such methods and apparatus are described in U.S. Pat. Nos. 5,576,747; 5,777,644 and 5,510,817. Methods and apparatus which employ an ink with electro-rheological properties to reduce the accelerating potential needed to discharge the ink are described in U.S. Pat. No. 5,576,747 and U.S. 5,510,817. Reducing the accelerating potential is said to facilitate miniaturization and simplification of the print head and reduce deformation of the print head by high temperature and pressure. In U.S. Pat. No. 5,777,644, an ink with electro-rheological properties is said to form channels within a print head to define the path of discharged ink. This technique is said to avoid clogging.
Electro-rheological fluids as described by Winslow in U.S. Pat. Nos. 2,417,850 and 3,047,507 are said to be of a minute particle diameter by Sohn, U.S. Pat. No. 5,576,747. As such, they are distinct from magneto-rheological fluids. Sohn, U.S. Pat. No. 5,549,837, also states that electro-rheological (ER) fluids exhibit lower yield strengths than magneto-rheological fluids and that the yield strength of electro-rheological fluids are sensitive to temperature. The higher yield strengths of magneto-rheological fluid enables these fluids to respond to low voltage/low current drive power supplies, which can be smaller in size.
Magnetic particles and ferrofluids have been incorporated in jet inks to provide MICR compositions (see U.S. Pat. No. 5,858,595). In contrast to MR fluids, ferro fluids consist of colloidal magnetic particles, such as magnetite, dispersed in a continuous carrier phase. The particles found in MR fluids are larger in size, typically greater than 0.1 .mu.m. Unlike MR fluids, ferrofluids do not solidify in an applied field, though they do exhibit field-induced viscosity increases. See J. P. McTague, J. Chem. Phys., 51, 133 (1969).
To operate satisfactorily in the processes and equipment used in ink jet printing, the ink must exhibit low viscosity values, contain no large particulate matter to clog the jet orifice and be sufficiently stable so as not to dry or settle such that the jet orifice does not clog when not in use. The ink must also dry rapidly and bond to the print substrate without smearing. Meeting all of these criteria can be complex where particulates are to be added to the jet ink.